Allow a value of 0 in pHYs chunks

[Oondanomala]

As far as I can tell that is allowed in the spec

[nayuki]

This is a bad idea because it can lead to division by zero.

For example, if we say that an image has "0 pixels per metre" and the image is 99 pixels wide, then its physical width is (99 pixel) / (0 pixel / metre) = (99 / 0) metre. I can't think of any acceptable use of the value zero here.

[Oondanomala]

Good point, though I could see it being used to indicate only one size is known (but then what if both sizes are 0? what if the unit is unknown?).
The spec doesn't seem to forbid that so I think they should be supported (or it should be updated to disallow it, though I don't see that happening).
I remember finding files in the wild that had a size of 0, which is why I made this PR in the first place (I don't remember what the files were though, I'll try to find them again).

Or maybe the spec should be updated to explicitly define 0 as unknown?